Without limiting the scope of the invention, its background is described in connection with composite materials.
Graphene is one of the strongest materials ever tested. Various research institutes have loaded hosts with carbon allotropes such as carbon nanotubes (CNT), graphene flakes (GF), graphene oxide (GO), and graphite oxide and have seen up to a 200% increase in tensile strength in the loaded host, but with inconsistent results. Measurements have shown that graphene has a breaking strength 200 times greater than steel, with a tensile modulus (stiffness) of 1 TPa (150,000,000 psi). An Atomic Force Microscope (AFM) has been used to measure the mechanical properties of a suspended graphene sheet. Graphene sheets held together by van der Waals forces were suspended over SiO2 cavities where an AFM tip was probed to test its mechanical properties. Its spring constant was in the range 1-5 N/m and the Young's modulus was 0.5 TPa (500 GPa) thereby demonstrating that graphene can be mechanically very strong and rigid.
Nano-silica's spherical shape, carbon nanotubes (CNTs) and Hummers' based GO that has both surface oxidation, epoxy groups and surface distortion have high-aspect-ratio. CNTs and Hummers' based graphene/graphite oxide are suspended and dispersed in water then combined with Ordinary Portland Cement and or other materials reacted to form a cementitious composite.
Depending on whether they are single walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs), generally have the diameter of 1-3 mm or 5-50 mm, respectively. The length of CNTs can be up to centimeters, which gives an aspect ratio exceeding 1000. CNTs also exhibit extraordinary strength with moduli of elasticity on the order of TPa and tensile strength in the range of GPa. With the concurrent benefits of high aspect ratio and excellent mechanical performance, CNTs have been found to improve the toughness and strength of cementitious matrix materials. Incorporation of CNTs in cement composites has proven to be complex, yielding inconsistent results. Researchers have found that the addition of CNTs results in little change in strength or even deterioration of the composite in some cases. Poor dispersion of CNTs in the water based cement matrix is due to the weak bonding between the CNTs and the cement matrix. Owing to strong Van der Waal's attractive forces between particles, CNTs tend to form agglomerates or self-attraction/assembly similar to that seen in carbon black creating defect sites in the composites. CNTs without a dispersing agent had worse mechanical properties than the plain cement paste. Non-uniform distribution/dispersion of CNT bundles are responsible for the deterioration of the mechanical properties.
Hummers' based graphite/graphene oxide (HGGO) is produced by using concentrated acids and oxidants and often required significant purification of the HGGO product. HGGO is more readily suspendable/dispersible in a polar solvent (ultra pure water) because the high level of oxidation making it hydrophilic. The HGGO can also irreversibly precipitate from the suspension in the polar solvent as a function of pH or exposure to mobile ions in a short period of time. This forces the suspension/dispersion of the HGGO to be done on site using tap water, done at a remote facility using ultra pure (DI) water or done at a remote facility and continuously mixed until it is used. If the HGGO suspension is improperly handled it will result in aggregation of the HGGO resulting in defects and damaging the physical properties in the composite. Additionally, strongly oxidized HGGO has mechanical distortions on the surface of the flake and epoxy groups due to the oxidation process. The physical distortions prevent optimal improvement of the physical properties in a cementations material. In addition to the physical damage to the graphene/graphite oxide flake the creation of epoxy groups prevent the formation of chemical reaction in the cementitious material.